This invention relates to closed loop air-fuel ratio control in internal combustion engines.
It is generally known that the amount of hydrocarbons, carbon monoxide and oxides of nitrogen emitted from an internal combustion engine may be substantially reduced by controlling the air-fuel ratio of the mixture admitted into the engine and catalytically treating the exhaust gases emitted therefrom. The optimum air-fuel ratio of the mixture supplied to the engine for most efficient reduction of the above described exhaust gas constituents is substantially the stoichiometric ratio. Even slight deviations from the stoichiometric ratio can cause substantial degradation in the reduction efficiency. Accordingly, it is important that precise control of the air-fuel ratio be maintained.
Conventional closed-loop air-fuel ratio control systems provide, by definition, feedback as to the actual air-fuel ratio of the mixture supplied to the engine, such as with the common zirconia oxide ZrO.sub.2 oxygen sensor disposed in the exhaust path of the engine. The ZrO.sub.2 sensor provides a high gain, substantially linear measurement of the oxygen content of the exhaust gas which, in a well known manner, may be translated into information on the actual ratio of fuel to air admitted into the engine. The translated information is used to make on-line corrections to the air-fuel ratio control. As such, it is important that accurate information on the actual air-fuel ratio be provided by the oxygen sensor.
Applicants have found that the ZrO.sub.2 sensor output predictably varies as the temperature of the sensor varies and as the frequency of the sensor varies. Accordingly, the accuracy of the feedback mechanism and, in turn, the accuracy of the air-fuel ratio tends to degrade as the temperature and switching frequency deviate away from a design temperature and switching frequency.
Conventional systems do not compensate for variations in ZrO.sub.2 sensor temperature and frequency and, as such, may be limited in their air-fuel ratio control accuracy.